Icahn’s investment, revealed in a filing with the Securities and Exchange Commission late Thursday, comes three weeks after a spat between the two men boiled over on live television, with each phoning in to CNBC and insulting each other.

The stock price of Herbalife rose $4.22, or 11 percent, to $42.50 Friday afternoon. It jumped more than 20 percent before the opening bell.

Icahn said in the regulatory filing that he plans to have discussions with Herbalife management about business and strategic alternatives to enhance shareholder value, including the possibility of going private.

That would be disastrous for investors who have shorted Herbalife, a tactic in which an investor “borrows” a share from a broker and sells it on the open market, anticipating the stock price will fall. Later, that share can be bought again at a lower price, locking in the difference for a profit.

In the stock keeps rising, short sellers could be forced to liquidate their positions to limit their losses. With the shares rising on Friday, many could do just that.

Ackman on Friday remained firm in his beliefs about Herbalife.

“After 18 months of due diligence, we have concluded that it is a certainty that Herbalife is a pyramid scheme,” he said. “Our goal was to shine a spotlight on Herbalife. To the extent that Mr. Icahn is helping achieve this objective, we welcome his involvement.”

The feud between Icahn and Ackman began a decade ago over a real estate company deal that ended up in court. Icahn was forced to pay $4.5 billion to Ackman’s Gotham Partners, and he has never forgotten about it.

Icahn has said publicly that he neither likes nor respects Ackman and didn’t like how he had approached his short position in Herbalife. He has previously refused to comment on whether he held a position in Herbalife. Many have speculated that he would take a stake in the company to strike at Ackman.

In an emailed response, a spokeswoman for Herbalife said, “We welcome all parties who see the same value in Herbalife that we do.”

Herbalife shares have been extremely volatile over the past few months. Greenlight Capital’s David Einhorn, another prominent Wall Street figure, raised concerns about the company’s business in May. And shares dropped sharply following a lengthy PowerPoint presentation by Ackman in December, when he declared that Herbalife was a pyramid scheme.

The company has disputed Ackman’s statements, saying that he is attacking it solely to strengthen his short position. Executives called a meeting with analysts and investors in January to detail how its business operates and who its customers are. Ackman replied that Herbalife “distorted, mischaracterized, and outright ignored large portions” of his presentation.